Double-fed asynchronous machines are usually used in plants which have a variable rotor speed, particularly in wind or water power plants. The use of an indirect converter which is connected to the rotor makes it possible to control the rotor voltage such that the frequency and phase position of the stator voltage remains constant in spite of a variable rotor speed. Thus, the stator can be connected to the mains directly or via a transformer. The relevant standards for the limits of harmonic system reactions necessitate the use of line filters for the double-fed system. As a function of the impedances of the mains and the design of the line filter, resonance characteristics of the controlled system arise which, in some cases, can lead to instability of the entire system of controlled system and control system which affects both the control system for the rotor-side converter and the control system for the line-side converter. The likelihood of such instabilities increases with an increasing number of individual filters connected in parallel and, particularly in the case of large wind farms, can result in the disconnection of plants and also in the destruction of components. In the prior art, to dampen such resonances, specifically configured line filters which are adapted to the respective plant site and have an unchangeable frequency-dependent damping are incorporated during delivery of the system or during installation thereof, such line filters being described, for example in Durstewitz et al., “Netzspezifische Filterauslegung”, Kasseler Symposium Energie-Systemtechnik 1998.
In a wind farm, the likelihood of an instability occurring changes if the wind farm is expanded by a new wind power plant or if a wind power plant is separated from the mains, e.g. for maintenance purposes. It is even possible for new resonances to arise in the system as a result of such changes. As a result, the line filters known hitherto from the art are no longer optimally adapted to the altered resonance characteristics of the system and no longer adequately dampen the resonances, thus entailing an increased likelihood of instabilities arising in the system.
If necessary, the line filter will even have to be removed and a new line filter with a fixed frequency-dependent damping adapted to the new resonance characteristics of the system will have to be installed. This is associated with high costs.